ld: library not found for -lomp

Question

In macOS Sierra, installation for xgboost with openmp enabled always fails.

From https://xgboost.readthedocs.io/en/latest/build.html,

I've tried:

cp make/config.mk ./config.mk; make -j4

With,

export CC=/usr/local/Cellar/llvm/4.0.0_1/bin/clang
export CXX=/usr/local/Cellar/llvm/4.0.0_1/bin/clang++
export CXX1X=/usr/local/Cellar/llvm/4.0.0_1/bin/clang++

It fails with,

clang-4.0clang-4.0: : warning: argument unused during compilation: '-pthread' [-Wunused-command-line-argument]warning:
argument unused during compilation: '-pthread' [-Wunused-command-line-argument]
ld: library not found for -lomp
ld: library not found for -lomp
clang-4.0: error: linker command failed with exit code 1 (use -v to see invocation)

llvm support openmp, but it fails with ld: library not found for -lomp

related question Install xgboost on Mac - ld: library not found

But, clang-omp goes to boneyard, and discontinue with llvm's OpenMP support.

I've drilled down to, (with -v option)

/usr/local/Cellar/llvm/4.0.0_1/bin/clang++ -std=c++11 -Wall -Wno-unknown-pragmas -Iinclude   -Idmlc-core/include -Irabit/include -I/include -O3 -funroll-loops -msse2 -fPIC -fopenmp -shared -o lib/libxgboost.so build/learner.o build/logging.o build/c_api/c_api.o build/c_api/c_api_error.o build/common/common.o build/common/hist_util.o build/data/data.o build/data/simple_csr_source.o build/data/simple_dmatrix.o build/data/sparse_page_dmatrix.o build/data/sparse_page_raw_format.o build/data/sparse_page_source.o build/data/sparse_page_writer.o build/gbm/gblinear.o build/gbm/gbm.o build/gbm/gbtree.o build/metric/elementwise_metric.o build/metric/metric.o build/metric/multiclass_metric.o build/metric/rank_metric.o build/objective/multiclass_obj.o build/objective/objective.o build/objective/rank_obj.o build/objective/regression_obj.o build/tree/tree_model.o build/tree/tree_updater.o build/tree/updater_colmaker.o build/tree/updater_fast_hist.o build/tree/updater_histmaker.o build/tree/updater_prune.o build/tree/updater_refresh.o build/tree/updater_skmaker.o build/tree/updater_sync.o dmlc-core/libdmlc.a rabit/lib/librabit.a -pthread -lm  -fopenmp

Answer 1

SOLVED

cd /usr/local/lib
ln -s /usr/local/Cellar/llvm/4.0.0_1/lib/libomp.dylib libomp.dylib

llvm installation miss its symbolic link.

Answer 2

In my case I solved adding the following linker flag in addition to -lomp:

-Lpath_to_libomp.dylib_folder

or something like:

-Llib/